The dissemination of metastatic cells from the primary tumor into the surrounding tissue is a key event in the progression of cancer. This process involves the migration of cells across defined tissue interfaces that separate the dense tumor tissue from the adjacent healthy tissue. Prior research showed that cell transmigration across collagen I matrix interfaces induces a switch towards a more aggressive phenotype including a change in directionality of migration and chemosensitivity correlated to increased DNA damage during transmigration. Hence, mechanical forces acting at the nucleus during transmigration are hypothesized to trigger phenotype switching. Here, we present results from a particle image velocimetry (PIV) based live cell analysis of breast cancer cell transmigration across sharp matrix interfaces constituted of two collagen type I networks with different pore sizes. We found strong and highly localized collagen network deformation caused by cellular forces at the moment of crossing interfaces from dense into open matrices. Additionally, an increased contractility of transmigrated cells was determined for cells with the switch phenotype. Moreover, studies on mechanotransductive signaling at the nucleus, emerin translocation and YAP activation, indicated a misregulation of these signals for transmigrated cells with altered phenotype. These findings show that matrix interfaces between networks of different pore sizes mechanically challenge invasive breast cancer cells during transmigration by a strong asymmetry of contracting forces, impeding nuclear mechanotransduction pathways, with a subsequent trigger of more aggressive phenotypes.